Amphibole peridotite and hornblendite of the Schrisheimite Hill intrusion of the Revdinskiy massif (Ural Platinum Belt)

Research subject . We studied the concentric-zonal massif (≈400 × 400 m) of amphibole-rich ultramafic rocks composed of amphibole peridotite (schriesheimite and amphibole wehrlite) with olivinite schlieren, pyroxenite and hornblendite located among gabbro at the Schrisheimite Hill in the Revdinskiy massif of the Ural Platinum Belt. The scope is characteristic of both of amphibole peridotites, which are extremely rare in the massifs of the Ural Platinum Belt, and the associated amphibole-rich ultramafic rocks. Aim. To expand the understanding of aqueous ultramafic magmatism in Ural Platinum Belt massifs. Materials and methods . To determine the age, we selected monofractions of high-alumina amphibole (pargasite-magnesiohastingsite) from two samples. Amphibole is not significantly altered by the regional metamorphism. 40 Ar/ 39 Ar dating was carried out according to the method described in (Travin et al., 2009; Yudin et al., 2021). The argon isotope composition was measured using a Micromass 5400 mass spectrometer (IGM SB RAS). Results. Amphibole peridotite has a hypidiomorphic (subhedral) or poikilitic texture. It is typical of igneous rocks, which indicates the crystallization of amphibole peridotite was by H 2 O-saturated magmas. Schlieren segregations of fine-grained schriesheimites and fine-grained olivinites are among coarse-grained schriesheimites. Amphibole peridotite, pyroxenite and hornblendite are deficient in high field strength elements (HFSE) and enriched in large ion lithophile elements (LILE) (with contrast concentration of Cs and Rb), which is typical of supra-subduction igneous formations. We obtained a 40 Ar/ 39 Ar age of 437.2 ± 6.7 Ma for an amphibole from hornblendite, which differs significantly from the ages of hornblendite determined earlier. Conclusions . Our new data indicate that hornblendite dikes in different UPB massifs formed over a fairly long time interval from the Early Silurian to the Early Devonian. These data expand modern ideas about the timing and nature of ultramafic water magmatism in the massifs of the Ural Platinum Belt.